Liquid crystalline display devices containing dichroic dyes are well known in the field of liquid crystal technology as display devices operable by a "guest-host" mode, and are utilized in watches, portable electronic calculators, televisions, etc.
The principle of these display devices lies in the fact that molecules of a dichroic dye and molecules of a liquid crystalline material are arranged such that the alignment of the dichroic dye molecules follows that of the molecules of the liquid crystalline material.
Specifically, they are based on the principle that by the application of an external stimulation which is usually an electric field, liquid crystal molecules change from an "off" state to an "on" state and are aligned, and incident to this, the dichroic dye molecules are simultaneously aligned, as a result of which the degree of light absorption by the dye molecules changes to display a color.
Such an electro-optical effect means liquid crystal color display which utilizes the so-called guest-host effect. The guest-host mode presently includes, for example, a method which involves using nematic liquid crystal having positive or negative dielectric anisotropy, and a method which involves using liquid crystals of the type which undergoes phase transition from a cholesteric phase to a nematic phase upon the application of an electric field. The present invention can be applied to any of these cases.
Generally, dichroic dyes used in liquid crystal color display devices to which such a principle is applied are basically required to meet such conditions as given below. (1) They should have a sufficient coloring ability in small amounts. (2) They should have a high dichroic ratio and show a high contrast by application and non-application of a voltage. (3) They should have sufficient compatibility with liquid crystals. (4) They should have excellent durability and be stable, and should not degrade the performance of the display devices even upon long-term use.
In recent years, various dyes having an anthraquinonic skeleton have been developed. Some of these dyes meet the aforesaid requirements, and contribute to the practical application of these display devices. However, most of these dichroic dyes having an anthraquinonic skeleton, because of their structural characteristics, exclusively show a red or blue color, and useful dichroic dyes having an anthraquinoic skeleton and showing a yellow color have not been obtained. As a result, one has to use azoic dyes having poor durability as yellow dichroic dyes. This markedly limits the range of application of the aforesaid display devices, and makes it impossible to meet the needs of the users sufficiently.
The present invention has been developed with a view to providing dichroic dyes having a yellow color which meet such requirements.